A universal problem in biology is the interpretation of the half-maximal response of a cell population to a stimulus. Either all the cells are producing only one-half of their maximal potential, or one half of the cells are producing at their full potential while the other half are producing nothing. To decide between these two alternatives we examined the production of interleukin 2 by single T cell clones at different concentrations of antigen. The response appeared to be quantal (all or more) but two different levels of response were observed, one approximately 3-4 times the other. Furthermore, the dose response curve was biphasic with a decrease in the number of cells in each group at higher (>30nM) antigen concentrations. Our interpretation of these results is that induction of transcription of the IL-2 gene is what is quantal. Thus, the group of cells giving the low level of IL-2 production had only one chromosome being transcribed, whereas the group giving the higher level of IL-2 production was transcribing both chromosomes. The fact that all the cells were not found in the high positive group at large antigen concentrations possible reflects a negative feedback regulation on IL-2 transcription. Experiments are currently in progress to test these two hypotheses.